(a) Field of the Invention
The present invention relates to a display device, more particularly, to a display device having a photo sensing function.
(b) Description of the Related Art
In recent years, lightweight and thin monitors or televisions are increasingly required or desired, and liquid crystal displays (“LCDs”) which are lightweight and thin are being substituted for conventional cathode ray tubes (“CRTs”).
However, since the LCD is a light receiving/emitting device, a separate backlight or light reflector is required. In addition, LCDs have many problems including slow response speed, a limited viewing angle, low contrast ratios, as well as other problems. Recently, an organic light emitting diode (“OLED”) display is attracting attention as a display device which can overcome these problems.
An OLED display includes two electrodes and a light emitting layer interposed therebetween. The light emitting layer combines electrons injected from one electrode and holes injected from the other electrode to form excitons, which emit light through energy emission. The OLED display is a display device which electrically excites a fluorescent organic material to emit light so as to display images. Since an OLED display is itself a light emitting display device which does not require a separate backlight or light reflector, it provides for lower power consumption, wider viewing angles and faster pixel response speeds, making it suitable to display high-quality motion pictures.
The organic light emitting diode (“OLED”) display includes organic light emitting elements (e.g., organic light emitting diodes, “OLEDs”) and thin film transistors (“TFTs”) which drive the organic light emitting elements. The (TFTs) are classified into several types including polysilicon TFTs, amorphous silicon TFTs, and so on, according to the type of active layer. An OLED display using the polysilicon thin film transistor has many merits and is widely used, but the manufacturing process of the polysilicon TFT is complex, which causes an increase in manufacturing costs. Also, a large screen cannot be achieved using such an OLED display.
Meanwhile, a large screen is easily achieved using an OLED display using amorphous silicon TFTs. Also, the manufacturing process of an OLED display using the amorphous silicon is much simpler when compared with an OLED display using the polysilicon thin film transistor.
However, as a positive DC voltage is continuously applied to a control terminal of the amorphous silicon TFT, the threshold voltage of the amorphous silicon thin film transistor changes. Even though a constant control voltage is applied to the TFTs, non-uniform currents flow within the OLEDs. For this reason, luminance of the OLED display is lowered, which causes degradation in image quality. Ultimately, the non-uniform currents within the OLEDs cause a reduction in a life span of the OLED display.
Up to now, various pixel circuits, which compensate for the variations of threshold voltages, have been suggested in order to prevent the degradation in image quality. However, most of the pixel circuits include many thin film transistors, capacitors and wiring, which results in a low pixel aperture ratio.
Accordingly, there remains a desire to prevent degradation in image quality by correcting a change in threshold voltage of an amorphous silicon TFT.